The process of using x-rays to replicate a pattern on a substrate is known as x-ray lithography. A mask made from an x-ray transparent material, such as boron nitride, having a thin pattern of gold thereon which is the negative image of the pattern of interest, is interposed between an x-ray source and a resist-coated substrate. When an x-ray beam is directed through the mask, the gold pattern on the boron nitride absorbs those x-rays impinging thereon. The x-rays impinging on the portions of the mask not covered by the gold pattern pass therethrough and strike the resist on the substrate. Once the resist is developed, the pattern is fixed on the substrate.
Using x-rays affords the advantage that particulates, which print on the substrate as defects during the lithography process when light in visible and ultraviolet (optical) wavelength range is used, become transparent when the substrate is exposed to radiation in the x-ray range. Further, diffraction effects result from using light in the optical wavelength range and limit the minimum achievable line width. Such diffraction effects are reduced by using x-ray radiation, allowing finer line widths to be achieved.
In the past, x-ray lithography has been practiced using electron impact sources which operate by directing electrons at a target material, which in turn radiates x-rays. Electron impact sources are not well suited for semiconductor x-ray lithography applications because the intensity of the x-rays produced thereby is low, requiring long exposures for each wafer, thereby limiting wafer throughput.
Moreover, electron impact sources have a large spot size, which is to say, if the x-ray source were assumed to be a sphere, it would have a large radius, typically on the order of 1.5 mm. X-ray sources having a large spot size incur the disadvantage that the x-rays produced thereby appear to originate over a wide area causing blurring of the pattern around the edges thereof which impedes the replication of very fine feature sizes on a substrate spaced a distance from the mask. The blurring of the pattern around the edges thereof due to the large spot size is referred to in the art as the penumbra effect.
Another type of x-ray source is the synchrotron which produces x-rays which are softer, that is to say of a lower energy, typically 1 keV, than those produced by electron impact sources. Synchrotron sources typically have a smaller, effective spot size as compared to electron impact sources, and thus do not suffer as much from the penumbra effect. However, synchrotron x-ray sources are not useful for routine x-ray lithographic applications because of their bulk and expense.
In addition to using impact and synchrotron x-ray sources, x-ray lithography can also be practiced using a plasma source. An example of an x-ray lithography system which utilizes a plasma x-ray source is disclosed in U.S. Pat. No. 4,184,078 issued to D. J. Nagel et al. on Jan. 15, 1980. The x-ray lithography system of Nagel et al. includes a plasma producing device, such as an exploding wire diode or gas injection diode situated in a vacuum chamber. The diode, when excited with a very large, time varying voltage from an energy source such as a capacitor bank, produces a hot, dense plasma within the chamber. The plasma radiates a burst of energy in the form of soft x-rays which are directed through a mask and onto a resist-coated substrate. For lithographic applications, gas injection and exploding wire diodes are undesirable plasma producing sources because they generate debris and are generally unpredictable in their operation.
An article "Gas Plasmas Yield X rays For Lithography" appearing at pages 40 and 41 of the Jan. 27, 1982 edition of Electronics magazine describes a plasma source for x-ray lithography applications. The source includes a nozzle through which a quantity of argon gas is puffed to assume the shape of a generally cylindrical shell. An intense electric field is applied to the shell to break down (ionize) the gas, transforming the shell into a sheath of plasma which passes a current axially along its surface. The current passing through the plasma generates a circumferential magnetic field which collapses the sheath, causing the plasma to become very hot and dense. The hot, dense plasma radiates a burst of energy in the form of soft x-rays. With this type of plasma x-ray source, the gas shell produced thereby is often nonuniform which may lead to unstable collapse of the plasma sheath, thereby affording poor control over the radiated x-rays.
Accordingly, there is a need for a technique for producing x-ray pulses suitable for lithographic applications.